Allow App Runner to return a 5xx error when scaling out

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**Tell us about your request**
What do you want us to build?

I am using SNS to deliver messages to App Runner. When App Runner reaches the concurrency limit it will return a number of 429 responses until the new container spins up to handle the traffic. SNS seems to treat all non-5xx responses as successful, so it does not try to re-deliver the message according to a retry policy, or put the message in a configured DLQ. Therefore, the message is lost.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

I have considered using SQS, but I prefer push messaging as much as possible. I think this (SNS to App Runner) is a valid use case for SNS since it is advertised as supporting application to application messaging.

**Additional context**
Anything else we should know?

If this is a wontfix because 429 is the correct response code (fair point), can someone get a hold of the SNS team and see if they can make a feature to allow an SNS subscription to treat 429 the same as it does for 5xx?

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